1. Field of the Invention
The present invention relates to a printed circuit board having electric or electronic parts soldered to its circuit patterns.
2. Related Art
Electric or electronic apparatuses use printed circuit boards, which have a variety of electric or electronic parts soldered to their circuit patterns. Specifically such circuit patterns are formed by labyrinth-like copper foils and the like, each having electric or electronic parts soldered to lands with their terminals inserted in the terminal holes, which are made in the lands.
FIG. 5(a) is a plan view of a land 51 whereas FIG. 5(b) is a sectional view of a land 51, showing a terminal 54 of a selected electric or electronic part soldered to the land 51. The land 51 is formed on the rear side (the upper side in FIG. 5(b) of a substrate 50, and the land 51 has a terminal hole 52 formed at its center. The land 51 is surrounded by a resist layer 53 of an insulating synthetic resin to prevent solder from attaching to the surrounding area. The terminal 54 of the electric or electronic part is joined to the land 51 by solder.
As seen from FIG. 5(b), a solder lump 55 adhered both to the land 51 and terminal 54 looks like a cone. Its inclined sides are dented, and it is small in volume or quantity compared with a hemisphere. Accordingly the cone-like solder lump 55 can hardly provide a holding strength sufficient to assure that the electric or electronic part is fixedly held on the printed circuit pattern. As a matter of fact, when electric or electronic devices having such printed circuit boards are transported, some parts are shaken loose from their printed circuit patterns. Also, printed circuit boards are influenced by surrounding temperature variations or by temperature changes caused by turning printed circuits on and off, so that some parts come off from their printed circuit patterns.
Therefore, it is necessary to locate small solder lumps for applying additional quantities of solder. This, however, is tedious and time-consuming work. Small solder lumps are easily overlooked. Also, it may be possible that an additional quantity of solder will cause short-circuiting.
In the hope of increasing the solder lump quantity on each land JP 2000-91737(A), titled “Printed Circuit Boards and Their Manufacturing Method” (see FIGS. 6(a) and 6(b)), has proposed use of a land comprising a center circular land and a plurality of second sector lands surrounding the first center land. The printed circuit board has electronic parts or devices borne with their terminals inserted in the terminal holes of the circular lands. Such a printed circuit board is moved to sweep on the melted solder bath, solder lumps separately built on the circular and sector lands, and then these solder lumps are joined together under the influence of surface tension to provide a large solder lump quantity.
FIG. 6(a) is a plan view of one example of such composite land whereas FIG. 6(b) is a cross section of the composite land taken along the line 6(b)—6(b) in FIG. 6(a). As shown, a center circular land 56 has a terminal hole 52 formed at its center, and four sector lands 57 surround the first center land 56. A resist layer 58 of an insulating synthetic resin is laid on the copper foil surface, filling the inter-land zones.
Printed circuit boards each having circuit patterns printed on their opposite surfaces are often used. These circuit patterns are locally connected via copper foils (conductor linings) applied to the inner surfaces of the terminal holes. With this arrangement heat can be transferred from the front to the rear circuit patterns via the conductor linings of the terminal holes when terminals of selected parts and devices are being soldered to one of the front and rear circuit patterns. Thus, the thermal quantity available for soldering will be lower than that for soldering in a printed circuit board having a circuit pattern on one surface. Division of the land into separate sections proposed in JP2000-91737(A) is not so effective in confining the thermal energy to the limited area for soldering, and hence, in increasing the solder quantities in joining the terminal and the land.
Further, JP8-340172(A), titled “Printed Circuit Boards” has proposed a composite land comprising a center circular land and a plurality of second radial lands projecting from the first center land. The inter-radial land spaces are filled with a resist layer of insulating synthetic resin, which is laid on the copper foil of the substrate. Therefore, thermal energy is permitted to leak from the composite land contour to the underlying copper foil of the inter-radial land space.
In view of the above, one object of the present invention is to provide a printed circuit board whose soldering structure permits an adequate quantity of solder to be used in soldering the terminal of the electric or electronic part to its circuit pattern, thus assuring that the solder lump be strong enough to fixedly hold the part or device on the circuit pattern. Also, another object of the present invention is to provide a soldering structure which permits an adequate quantity of solder to be used in soldering the terminal of the electric or electronic part or device to its circuit pattern, thus assuring that the solder lump be strong enough to fixedly hold the part or device on the circuit pattern.